1. Field of the Invention
The present invention relates to a transversal type filter utilizing an elastic wave such as a SAW (Surface Acoustic Wave).
2. Description of the Related Art
A SAW device utilizes surface acoustic waves, and by arranging electrodes called an IDT (inter-digital transducer) on a piezoelectric substrate to perform an electromechanical interconversion between an electrical signal and an elastic wave, it provides frequency selection (band filter) characteristics. A SAW filter, which is one of such SAW devices, is used as a band-pass filter for various communication devices such as, for instance, a television broadcast transmitting station. In recent years, according to the decision of a shift from analog broadcasting to digital broadcasting in the year 2011, the band-pass filter for the digital broadcast transmitting station has been required to have a wide frequency pass band, as its characteristic. Further, this filter is required to have a high flatness of a signal level and a high selectivity (steep attenuation gradient) in the frequency pass band compared to a standard of another point-to-point communication band-pass filter, as shown in FIG. 15. Accordingly, a transversal type filter is generally used as the filter, since it can independently design an amplitude and a phase of a frequency characteristic.
A filter 100 is provided with an input (transmitting side) IDT 102 and an output (receiving side) IDT 103 formed on a piezoelectric substrate 101, as shown in FIG. 16(a), for instance. The input IDT 102 and the output IDT 103 are formed so as to face each other in a traveling direction of an elastic wave, and each includes a pair of bus bars 104, 104 formed so as to be parallel to a propagation direction of the elastic wave, and a large number of finger electrodes 105 alternately extending from the bus bars 104, 104 in a comb-teeth shape in a direction orthogonal to the propagation direction of the elastic wave. Note that dampers 106, 106 formed on an outside of the input IDT 102 and the output IDT 103 (both ends of the piezoelectric substrate 101 in the longitudinal direction thereof) serve to absorb unnecessary elastic waves propagated from the input IDT 102 and the output IDT 103. Further, a not-shown shield electrode to suppress coupling between the input IDT 102 and the output IDT 103 is formed between the input IDT 102 and the output IDT 103.
Either of the input IDT 102 and the output IDT 103, which is, for instance, the output IDT 103 is formed as a normal type electrode in which an aperture L being a aperture between the alternately extending finger electrodes 105, 105 is formed to be as long as possible, namely, a gap D between a tip of the finger electrode 105 and the bus bar 104 is formed to be as small as possible, and the aperture L is formed to take a constant value in the respective finger electrodes 105, as shown in FIG. 17(a) in a schematic manner.
Meanwhile, the other of the input IDT 102 and the output IDT 103, which is, for instance, the input IDT 102 is formed as an apodized type electrode in which the aperture L is formed to be continuously changed in the propagation direction of the elastic wave, as shown in FIG. 17(b) in a schematic manner. Concretely, as shown in the aforementioned FIG. 16(a), the finger electrodes 105 in the input IDT 102 are formed so that they have a width A in which the aperture L takes the maximum value, at a center portion in the propagation direction of the elastic wave, and the apertures L of both sides of the width A become gradually small until they reach, for instance, almost zero. Accordingly, it can be said that in the region, a figure of schematically oval shape is drawn by a large number of gaps D. If the region of schematically oval shape is called as a main lobe M, regions in which the finger electrodes 105 are formed to have a maximum aperture L whose size is smaller than the width A and is, for instance, a width B (referred to as side lobes S1) are disposed on both sides of the main lobe M. On an outside of the side lobes S1, S1 (end portion sides of the electrode 102), side lobes S2, S2 each having the maximum aperture L smaller than the width B are formed in the same manner, and although an illustration is omitted, on an outside of the side lobes S2, S2, side lobes S3, S4, . . . , Sn in which their maximum apertures L become gradually smaller than the maximum aperture L of the side lobe S2, are disposed. Note that the gaps D are sequentially and alternately formed from upper and lower parts in the respective lobes in the propagation direction of the elastic wave, in which on a boundary between the respective lobes, two gaps D are continuously disposed on either of upper and lower parts in the lobe to reverse the order of the gaps D, to thereby invert a phase of the elastic wave generated between the adjacent lobes.
In the filter 100, when an electrical signal is input into the input IDT 102, an electric field is formed each between the finger electrodes 105, 105 which cross in the input IDT 102 and the piezoelectric substrate 101 is distorted, resulting that the elastic wave, for instance, a SAW (Surface Acoustic Wave) is generated. The electric field is formed by a tap being a crossing region between the finger electrodes 105, 105, so that in the input IDT 102, a large number of elastic waves each having an intensity in accordance with the respective apertures L are generated. These elastic waves are propagated toward, for instance, the output IDT 103, in which a distance between the position of the tap in which the elastic wave is generated and the output IDT 103, for instance, a left end of the output IDT 103 differs depending on each of the elastic waves. Accordingly, the elastic wave generated at a right end of the input IDT 102 first reaches the output IDT 103, and thereafter, the elastic waves generated at the left side of the right end sequentially reach the output IDT 103.
Subsequently, these elastic waves are converted into electrical signals in the output IDT 103, and accordingly, a signal (time response) whose intensity is continuously changed in accordance with the elapse of time is obtained. Therefore, the time response is represented by a waveform having an amplitude intensity corresponding to the aperture L from the left end of the input IDT 102 to the right end thereof, as shown on the right side of FIG. 16(b). As above, in the input IDT 102, the finger electrodes 105 are weighted by changing the respective apertures L, to thereby obtain the time response having a desired amplitude intensity. Note that in the output IDT 103, a part of the elastic waves propagated from the input IDT 102 is received to be converted into the electrical signal, and the remaining elastic waves are reflected by the output IDT 103 to return to the input IDT 102. Subsequently, the elastic waves returned to the input IDT 102 are reflected by the input IDT 102 to propagate to the output IDT 103 again, a part of the propagated elastic waves is received by the output IDT 103 to be converted into the electrical signal, and the remaining elastic waves are reflected toward the input IDT 102. As above, the elastic waves are repeatedly reflected between the input IDT 102 and the output IDT 103 until all energies thereof are converted into the electrical signals in the output IDT 103 or until they are propagated to the end portion sides of the piezoelectric substrate 101 via the input IDT 102 and the output IDT 103 and absorbed by the dampers 106.
Subsequently, by performing a Fourier transform on the time response, a frequency response in which a specific frequency region is extracted is obtained, as shown on the left side of FIG. 16(b). At this time, an amplitude intensity of frequency extracted at the output IDT 103 (insertion loss of the filter 100) is changed depending on a weighting value of each lobe in the input IDT 102. Meanwhile, as the weighting value becomes smaller than a predetermined threshold value, namely, as the elastic wave positions further toward, for instance, the end portion side of the input IDT 102, the influence of the elastic wave excited in the region on the flatness (“C” in FIG. 16(b)) and the attenuation characteristic (“D” in FIG. 16(b)) in the band becomes greater than that on the amplitude intensity of frequency.
In order to widen the frequency pass band as described above in such a filter 100, a method of increasing the number of side lobes S (the number of taps), for instance, is generally applied. However, since the maximum aperture L becomes gradually small from the main lobe M toward the side lobe Sn as described above, if the number of side lobes S is increased, the apertures L at the both end portions of the input IDT 102 become quite small. Therefore, as shown in FIGS. 18(a) and 18(b), when the elastic wave is output from, for instance, the right end of the input IDT 102, it is largely expanded in the direction orthogonal to the propagation direction of the elastic wave due to a diffraction. Further, the elastic wave generated at, for instance, the left end of the input IDT 102 is also largely expanded in the direction orthogonal to the propagation direction of the elastic wave similarly due to the diffraction, when it propagates through the input IDT 102 toward the right side. Further, if the elastic wave is propagated to, for example, a region deviated in the longitudinal direction from the region between the bus bars 104, 104 of the output IDT 103 due to the diffraction, the elastic wave that is not received is generated, which results in loss of energy. As a result, as shown by a dotted line in FIG. 19, the flatness and the attenuation characteristic are deteriorated compared to a case in which no diffraction is generated (solid line). Note that a propagation distance of the elastic wave to the output IDT 103 becomes greater as the elastic wave positions further toward the left side of the input IDT 102 as described above, so that a diffraction loss at the left end of the input IDT 102 becomes greater than that at the right end of the input IDT 102.
Meanwhile, as a method of performing the weighting on the finger electrodes 105, a withdrawal method in which a plurality of finger electrodes 105 are continuously formed from, for instance, one of the bus bars 104 (bus bar 104 at the lower side, in this example) so that an interval (position) at which the elastic wave is generated is adjusted as shown in, for example, FIG. 20(a), is known other than the aforementioned apodizing method. Apodizing method is one of weighting IDT in which a aperture of finger electrodes is changed in a propagation direction of the elastic wave. Further, there is also known a method such as a dog-leg method in which floating electrodes 107 extending in parallel with the bus bars 104 are disposed at positions in which the aperture L becomes 1/n (n: positive number) to divide a propagation region being a region between the bus bars 104, 104 into tracks being n propagation paths, finger electrodes 108 extending in a direction orthogonal to the bus bars 104 are formed on the floating electrodes 107, and the finger electrodes 105 (108) are alternately disposed in each track, to thereby make an amplitude of the generated elastic wave into 1/n, as shown in FIG. 20(b). With the use of these methods, it is possible to obtain a long aperture L compared to a case in which the aforementioned apodizing method is used, so that even when the weighting is performed so as to reduce the amplitude of the elastic wave, it is possible to suppress the diffraction of the elastic wave.
However, since the withdrawal method performs the weighting depending on the presence/absence of the finger electrodes 105, the weighting cannot be conducted continuously as in the apodizing method and the weighting amount becomes discrete, so that it lacks an expression of characteristic. Accordingly, the application of only this method is not suitable for realizing the widening of the frequency pass band, the improvement of flatness of the signal level or the improvement of attenuation characteristic (enhancement of the selectivity) in the frequency pass band. Further, the dog-leg method performs the weighting according to the number of divisions of the propagation region (the number of tracks), so that also in this method, the weighting amount becomes discrete. In addition, the floating electrode 107 is not connected to the outside (input port, output port or ground), so that even if the aperture between the finger electrodes 105 (108) is the same, there is generated a difference, although very little, between an electric field (k1) excited between the finger electrode 105 and the finger electrode 108 and an electric field (k2) excited between the finger electrode 108 and the finger electrode 108. Therefore, in order to suppress a mutual interference between the elastic waves generated in these electric fields, it is preferable to provide a gap K in which, for instance, the finger electrodes 105 (108) are not formed and a weighting value is forcibly set to zero between regions having a different number of divisions. Accordingly, if a large number of divided regions are formed, namely, if regions having different weighting amounts are increased, the gaps K are increased, resulting that unnecessary undulation is generated in the band, as shown in FIG. 21.
The weighting methods described above have advantages and disadvantages as shown in FIG. 22, and each method is inadequate to satisfy the aforementioned standard. Further, there is known a method of performing the weighting on the finger electrodes 105 of both the input IDT 102 and the output IDT 103 using either of the above-described methods, or a method of suppressing the diffraction loss by weighting a region close to the main lobe M of either of the IDT 102 (103) using the apodizing method and weighting a region in proximity to the side lobe Sn, namely, an end portion of the electrode 102 (103) using the withdrawal method, but, either of the methods is inadequate to satisfy the aforementioned standard. Patent Documents 1 to 6 disclose the weighting method such as the dog-leg method, but, no studies have been done regarding the above-described problems.
[Patent Document 1] Japanese Patent Application Laid-open No. 2004-320714
[Patent Document 2] Japanese Patent Application Laid-open No. Sho 56-132807
[Patent Document 3] Japanese Patent Application Laid-open No. Hei 5-29874
[Patent Document 4] Japanese Patent Application Laid-open No. Hei 5-183371
[Patent Document 5] Japanese Patent Application Laid-open No. Hei 7-50548
[Patent Document 6] Japanese Patent Application Laid-open No. Hei 10-303692